Contacts can be grouped together for quick reference, and can be managed as follows:
Organize contacts by group
Tap 'All Contacts' in the upper-left of the all contacts view to see which groups belong to your accounts. You can tap the groups to show or hide them in the all contacts view.
Tap the Add Group button to create a new group for that particular account. Tap the Edit button in the upper-right to delete or rename your groups.
Assigning contacts to groups
Tap and hold on a contact name to add it to a group or move it from one group to another. Open a contact's card to see which groups they currently belong to. You can remove contacts from existing groups in the contact card too.
Using pre-configured smart groups
Tap the Add Smart Group button to choose from a list of preconfigured smart groups to automatically filter your contacts.
Syncing smart groups from Cardhop for Mac
Cardhop for Mac allows you to create smart groups that auto-populate the group based on specific parameters. At present, Cardhop of iOS does not allow you to create new smart groups. However any existing smart groups you've created on Cardhop for Mac will sync to Cardhop for iOS automatically, providing both devices are signed into the same iCloud account.